As a drug-releasing controlling membrane (a drug permeation membrane) used in a plaster preparation for a transdermal administration, a polymer membrane of an ethylene vinyl acetate copolymer, an acrylic resin, a polyethylene, ethyl cellulose, etc., and a porous membrane thereof, or a membrane of gelatin which is a natural material have been known hitherto. The gelatin membrane includes a porous membrane which is prepared by adding a crosslinking agent to an aqueous solution of gelatin and an aqueous solution of dextran to form a membrane and, after drying, soaking the membrane into water to remove dextran; a membrane prepared by adding glycerin and a crosslinking agent to an aqueous solution of gelatin to form a membrane, followed by drying; and the like. These gelatin membranes are not irritative to the skin and do not cause inflammation and, from this standpoint, are considered to be superior to the above-described polymer membrane.
However, with respect to these gelatin membranes, it is difficult to control the rate of transfer of a minute amount of a certain type of drugs in the membrane and to control its amount to be permeated. With respect to the above-described polymer membrane, these controls have been found more difficult.
Since some of the drugs for transdermal administration are required to be absorbed gradually into the skin in a minute amount over a long period of time. In this case, it is necessary to strictly control the amount absorbed into the skin. Accordingly, the membrane which is unable to strictly control the rate and the permeating amount of a minute amount of the drug, such as the conventional gelatin membrane and polymer membrane, has a problem that it cannot be used as a releasing controlling membrane (a permeation membrane) for the drug which requires the above-described controls.
The present invention has been achieved in the light of the above-described problems, and an object thereof is to provide a phase-separated membrane which is capable of easily controlling a permeation rate and the permeation amount of a minute amount of drugs or other chemical substances.